There is known a laser radar device or a following-distance sensor that measures a following distance to preceding vehicle using laser beams. Here, the laser radar device emits laser beams forward of a subject vehicle and receives beams reflected from the preceding vehicle. The device thereby determines the distance or orientation to the preceding vehicle based on the received beams. Using the determined distance or orientation, vehicle controls such as following-distance control to the preceding vehicle are conducted.
This conventional device lowers its detection capability or measurement capability when dirt is attached to device's portions for emitting or receiving laser beams. A system where a vehicle control is performed using the measured results is stopped when the measurement capability (or measurable distance) is lowered. (Refer to Patent Document 1.) This poses a problem that the device has a low working ratio since the system is simply cancelled when the measurement capability becomes low.                Patent Document 1: JP-2002-196075 A        